1. Field of the Invention
The present invention relates to a film carrier employed in mounting IC chips, and particularly to a film carrier preferably used for TAB (Tape Automated Bonding).
2. Description of the Related Art
TAB is one of the known IC mounting techniques as disclosed in "MICRO ELECTRONICS PACKAGING HANDBOOK, P423, Edited by Rao R. Tummala et al. VAN NOSTRAND, 1989". In conventional TAB, IC chips are fixed through many conductive leads formed in a predetermined pattern on a film carrier comprising a long film substrate made of insulating material such as polyimide resin. A support ring made of the same material as the film substrate is formed around each IC chip to support the conductive leads. The IC chip is sealed (packaged) with resin using a potting method.
If upper and lower resin layers are not sufficiently set together in sealing the IC chip, the resin layer may be separated or cracked in later processing stages. If a boundary between the sealing resin (usually epoxy resin) and the other parts such as a polyimide film substrate of the support ring and the IC chip is exposed, moisture may penetrate the resin through the boundary to crack the resin, or the difference between thermal expansion coefficients of the materials around the boundary may create cracks in the resin.
To cope with this, a transfer mold method has been recently employed to perfectly seal the support ring and the IC chip. This method can solve the above problems and accurately define the shape and size of an IC package.
To sufficiently fill a mold employed by the transfer mold method with resin and to let the resin sufficiently penetrate gaps between outer lead portions of the conductive leads, the resin must have a low viscosity.
If the IC chip packaged on the carrier film has many pins, a gap between the adjacent leads will be very narrow. The resin will then ooze due to capillarity between the outer lead portions. This extremely degrades the appearance and commercial value of the IC package. In addition, the oozed resin may be carbonized in later processing stages to reduce or eliminate the insulation between the leads.
This sort of ooze also occurs in the potting method.
To deal with this problem, metal molds at least one of which has a surface matching the outer lead portions are used so that, when the metal molds are joined together at the bases of the outer lead portions, the metal molds will fill gaps between the outer lead portions, thereby preventing the resin from oozing.
This method, however, requires complicated metal molds which increase the cost of the packaging process. In addition, if patterns of the leads are changed, new metal molds must be prepared for the changed patterns.